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Requirements Elicitation

2Bernd Bruegge & Allen H. Dutoit Object-Oriented Software Engineering: Using UML, Patterns, and Java 2

Software Lifecycle Activities

Application

Domain Objects

Subsystems

class...class...class...

Solution Domain Objects

SourceCode

Test Cases

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Expressed in Terms Of

Structured By

Implemented

ByRealized By Verified By

SystemDesign

DetailedDesign

Implemen-tation

Testing

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RequirementsElicitation

Use CaseModel

Analysis


What does the Customer say?


First step in identifying the Requirements: System identification• Two questions need to be answered:

1. How can we identify the purpose of a system? 2. What is inside, what is outside the system?

• These two questions are answered during requirements elicitation and analysis

• Requirements elicitation:• Definition of the system in terms understood by the

customer (“Requirements specification”)

• Analysis: • Definition of the system in terms understood by the

developer (Technical specification, “Analysis model”)

• Requirements Process: Contains the activities Requirements Elicitation and Analysis.


Techniques to elicit Requirements

• Bridging the gap between end user and developer:

• Questionnaires: Asking the end user a list of pre-selected questions

• Task Analysis: Observing end users in their operational environment

• Scenarios: Describe the use of the system as a series of interactions between a concrete end user and the system

• Use cases: Abstractions that describe a class of scenarios.


Scenario-Based Design

Scenarios can have many different uses during the software lifecycle

• Requirements Elicitation: As-is scenario, visionary scenario

• Client Acceptance Test: Evaluation scenario• System Deployment: Training scenario

Scenario-Based Design: The use of scenarios in a software lifecycle activity


Types of Scenarios

• As-is scenario:• Describes a current situation. Usually used in re-

engineering projects. The user describes the system • Example: Description of Letter-Chess

• Visionary scenario:• Describes a future system. Usually used in greenfield

engineering and reengineering projects • Can often not be done by the user or developer alone

• Example: Description of an interactive internet-based Tic Tac Toe game tournament

• Example: Description - in the year 1954 - of the Home Computer of the Future.


Additional Types of Scenarios (2)

• Evaluation scenario:• Description of a user task against which the system is

to be evaluated.• Example: Four users (two novice, two experts) play

in a TicTac Toe tournament in ARENA.

• Training scenario:• A description of the step by step instructions that guide

a novice user through a system• Example: How to play Tic Tac Toe in the ARENA

Game Framework.


How do we find scenarios?

• Don’t expect the client to be verbal if the system does not exist

• Client understands problem domain, not the solution domain.

• Don’t wait for information even if the system exists

• “What is obvious does not need to be said”

• Engage in a dialectic approach • You help the client to formulate the requirements• The client helps you to understand the requirements• The requirements evolve while the scenarios are being

developed


Heuristics for finding scenarios• Ask yourself or the client the following questions:

• What are the primary tasks that the system needs to perform?

• What data will the actor create, store, change, remove or add in the system?

• What external changes does the system need to know about?

• What changes or events will the actor of the system need to be informed about?

• However, don’t rely on questions and questionnaires alone

• Insist on task observation if the system already exists (interface engineering or reengineering)

• Ask to speak to the end user, not just to the client• Expect resistance and try to overcome it.


Scenario example: Warehouse on Fire

• Bob, driving down main street in his patrol car notices smoke coming out of a warehouse. His partner, Alice, reports the emergency from her car.

• Alice enters the address of the building into her wearable computer , a brief description of its location (i.e., north west corner), and an emergency level.

• She confirms her input and waits for an acknowledgment.• John, the dispatcher, is alerted to the emergency by a

beep of his workstation. He reviews the information submitted by Alice and acknowledges the report. He allocates a fire unit and sends the estimated arrival time (ETA) to Alice.

• Alice received the acknowledgment and the ETA.


Observations about Warehouse on Fire Scenario

• Concrete scenario• Describes a single instance of reporting a fire

incident.• Does not describe all possible situations in

which a fire can be reported.

• Participating actors• Bob, Alice and John


After the scenarios are formulated

• Find all the use cases in the scenario that specify all instances of how to report a fire

• Example: “Report Emergency“ in the first paragraph of the scenario is a candidate for a use case

• Describe each of these use cases in more detail • Participating actors• Describe the entry condition • Describe the flow of events • Describe the exit condition • Describe exceptions• Describe nonfunctional requirements


Requirements Elicitation: Difficulties and Challenges

• Communicate accurately about the domain and the system

• People with different backgrounds must collaborate to bridge the gap between end users and developers

• Client and end users have application domain knowledge

• Developers have solution domain knowledge

• Identify an appropriate system (Defining the system boundary)

• Provide an unambiguous specification• Leave out unintended features


Example of an Ambiguous Specification

During a laser experiment, a laser beam was directed from earth to a mirror on the Space Shuttle Discovery

The laser beam was supposed to be reflected back towards a mountain top 10,023 feet high

The operator entered the elevation as “10023”

The light beam never hit the mountain topWhat was the problem?

The computer interpreted the number in miles...


From the News: London underground train leaves station without driver!

Example of an Unintended Feature

• He left the driver door open• He relied on the specification that said the train

does not move if at least one door is open

• The driver left his train to close the passenger door

• When he shut the passenger door, the train left the station without him•The driver door was not treatedas a door in the source code!

What happened?• A passenger door was stuck and did not close


Requirements Process:problem

statement

Requirementselicitation

Analysis Model

RequirementsSpecification

:dynamic model

:analysis objectmodel

Analysis

:nonfunctionalrequirements

:functionalmodel


Requirements Specification vs Analysis Model

Both focus on the requirements from the user’s view of the system

• The requirements specification uses natural language (derived from the problem statement)

• The analysis model uses a formal or semi-formal notation (we use UML)


Types of Requirements

• Functional requirements• Describe the interactions between the system and its

environment independent from the implementation

“An operator must be able to define a new game. “

• Nonfunctional requirements• Aspects not directly related to functional behavior.

“The response time must be less than 1 second”

• Constraints• Imposed by the client or the environment

• “The implementation language must be Java “• Called “Pseudo requirements” in the text book.


Functional vs. Nonfunctional Requirements

Functional Requirements• Describe user tasks

that the system needs to support

• Phrased as actions“Advertise a new league”

“Schedule tournament”

“Notify an interest group”

Nonfunctional Requirements• Describe properties of the

system or the domain• Phrased as constraints or

negative assertions“All user inputs should be

acknowledged within 1 second”

“A system crash should not result in data loss”.


Types of Nonfunctional Requirements

• Usability• Reliability

• Robustness• Safety

• Performance• Response time• Scalability • Throughput• Availability

• Supportability• Adaptability• Maintainability

• Implementation• Interface• Operation• Packaging• Legal

• Licensing (GPL, LGPL)• Certification• Regulation

Quality requirementsConstraints or

Pseudo requirements


Nonfunctional Requirements: Examples

• “Spectators must be able to watch a match without prior registration and without prior knowledge of the match.”Usability Requirement

• “The system must support 10 parallel tournaments”Performance Requirement

• “The operator must be able to add new games without modifications to the existing system.”Supportability Requirement


What should not be in the Requirements?

• System structure, implementation technology• Development methodology

• Parnas, How to fake the software development process

• Development environment• Implementation language• Reusability

• It is desirable that none of these above are constrained by the client. Fight for it!


Requirements Validation

Requirements validation is a quality assurance step, usually performed after requirements elicitation or after analysis

• Correctness: • The requirements represent the client’s view

• Completeness: • All possible scenarios, in which the system can be used,

are described

• Consistency:• There are no requirements that contradict each other.


Requirements Validation (2)

• Clarity:• Requirements can only be interpreted in one way

• Realism: • Requirements can be implemented and delivered

• Traceability:• Each system behavior can be traced to a set of

functional requirements

• Problems with requirements validation: • Requirements change quickly during requirements

elicitation• Inconsistencies are easily added with each change• Tool support is needed!


We can specify Requirements for “Requirements Management”

• Functional requirements:• Store the requirements in a shared repository

• Provide multi-user access to the requirements • Automatically create a specification document

from the requirements • Allow change management of the

requirements • Provide traceability of the requirements

throughout the artifacts of the system.


Tools for Requirements Management (2)

DOORS (Telelogic)

• Multi-platform requirements management tool, for teams working in the same geographical location. DOORS XT for distributed teams

RequisitePro (IBM/Rational)

• Integration with MS Word• Project-to-project comparisons via XML baselines

RD-Link (http://www.ring-zero.com)

• Provides traceability between RequisitePro & Telelogic DOORS

Unicase (http://unicase.org)• Research tool for the collaborative development of

system models • Participants can be geographically distributed.


Different Types of Requirements Elicitation

• Greenfield Engineering• Development starts from scratch, no prior system

exists, requirements come from end users and clients• Triggered by user needs

• Re-engineering• Re-design and/or re-implementation of an existing

system using newer technology• Triggered by technology enabler

• Interface Engineering• Provision of existing services in a new environment• Triggered by technology enabler or new market needs


Prioritizing requirements

• High priority• Addressed during analysis, design, and implementation• A high-priority feature must be demonstrated

• Medium priority• Addressed during analysis and design• Usually demonstrated in the second iteration

• Low priority • Addressed only during analysis• Illustrates how the system is going to be used in the

future with not yet available technology


Requirements Analysis Document Template1. Introduction2. Current system3. Proposed system

3.1 Overview3.2 Functional requirements3.3 Nonfunctional requirements3.4 Constraints (“Pseudo requirements”) 3.5 System models

3.5.1 Scenarios3.5.2 Use case model3.5.3 Object model 3.5.3.1 Data dictionary 3.5.3.2 Class diagrams3.5.4 Dynamic models3.5.5 User interface

4. Glossary


Scenario example from earlier: Warehouse on Fire

• Bob, driving down main street in his patrol car notices smoke coming out of a warehouse. His partner, Alice, reports the emergency from her car.

• Alice enters the address of the building into her wearable computer , a brief description of its location (i.e., north west corner), and an emergency level.

• She confirms her input and waits for an acknowledgment.• John, the dispatcher, is alerted to the emergency by a

beep of his workstation. He reviews the information submitted by Alice and acknowledges the report. He allocates a fire unit and sends the estimated arrival time (ETA) to Alice.

• Alice received the acknowledgment and the ETA.


Observations about Warehouse on Fire Scenario

• Concrete scenario• Describes a single instance of reporting a fire

incident.• Does not describe all possible situations in

which a fire can be reported.

• Participating actors• Bob, Alice and John


After the scenarios are formulated

• Find all the use cases in the scenario that specify all instances of how to report a fire

• Example: “Report Emergency“ in the first paragraph of the scenario is a candidate for a use case

• Describe each of these use cases in more detail • Participating actors• Describe the entry condition • Describe the flow of events • Describe the exit condition • Describe exceptions• Describe nonfunctional requirements


Use Case Model for Incident Management

ReportEmergency

FieldOfficer DispatcherOpenIncident

AllocateResources

<<initiates>><<initiates>>

<<initiates>>


How to find Use Cases

• Select a narrow vertical slice of the system (i.e. one scenario)

• Discuss it in detail with the user to understand the user’s preferred style of interaction

• Select a horizontal slice (i.e. many scenarios) to define the scope of the system.

• Discuss the scope with the user

• Use illustrative prototypes (mock-ups) as visual support

• Find out what the user does• Task observation (Good)• Questionnaires (Bad)


Use Case Example: ReportEmergency

• Use case name: ReportEmergency• Participating Actors:

• Field Officer (Bob and Alice in the Scenario)• Dispatcher (John in the Scenario)

• Exceptions:• The FieldOfficer is notified immediately if the

connection between terminal and central is lost.• The Dispatcher is notified immediately if the

connection between a FieldOfficer and central is lost.

• Flow of Events: on next slide.• Special Requirements:

• The FieldOfficer’s report is acknowledged within 30 seconds. The selected response arrives no later than 30 seconds after it is sent by the Dispatcher.


Use Case Example: ReportEmergencyFlow of Events

1. The FieldOfficer activates the “Report Emergency” function of her terminal. FRIEND responds by presenting a form to the officer.

2. The FieldOfficer fills the form, by selecting the emergency level, type, location, and brief description of the situation. The FieldOfficer also describes a response to the emergency situation. Once the form is completed, the FieldOfficer submits the form, and the Dispatcher is notified.

3. The Dispatcher creates an Incident in the database by invoking the OpenIncident use case. He selects a response and acknowledges the report.

4. The FieldOfficer receives the acknowledgment and the selected response.


Order of steps when formulating use cases

• First step: Name the use case• Use case name: ReportEmergency

• Second step: Find the actors• Generalize the concrete names (“Bob”) to participating

actors (“Field officer”)• Participating Actors:

• Field Officer (Bob and Alice in the Scenario)• Dispatcher (John in the Scenario)

• Third step: Concentrate on the flow of events• Use informal natural language


Use Case Associations

• Dependencies between use cases are represented with use case associations

• Associations are used to reduce complexity• Decompose a long use case into shorter ones• Separate alternate flows of events• Refine abstract use cases

• Types of use case associations• Includes• Extends• Generalization


<<include>>: Functional Decomposition• Problem:

• A function in the original problem statement is too complex

• Solution: • Describe the function as the aggregation of a set of

simpler functions. The associated use case is decomposed into shorter use cases

ManageIncident

CreateIncident HandleIncident CloseIncident

<<include>><<include>> <<include>>


<<include>>: Reuse of Existing Functionality• Problem: There are overlaps among use cases. How

can we reuse flows of events instead of duplicating them?

• Solution: The includes association from use case A to use case B indicates that an instance of use case A performs all the behavior described in use case B (“A delegates to B”)

• Example: Use case “ViewMap” describes behavior that can be used by use case “OpenIncident” (“ViewMap” is factored out)

ViewMapOpenIncident

AllocateResources

<<include>>

<<include>>

Base UseCase

SupplierUse Case


<<extend>> Association for Use Cases• Problem: The functionality in the original problem

statement needs to be extended.• Solution: An extend association from use case A

to use case B• Example: “ReportEmergency” is complete by

itself, but may be extended by use case “Help” for a scenario in which the user requires help

ReportEmergency

FieldOfficerHelp

<<extend>>


Generalization in Use Cases• Problem: We want to factor out common (but not

identical) behavior. • Solution: The child use cases inherit the behavior

and meaning of the parent use case and add or override some behavior.

• Example: “ValidateUser” is responsible for verifying the identity of the user. The customer might require two realizations: “CheckPassword” and “CheckFingerprint”

ValidateUserParentCase

ChildUse Case

CheckPassword

CheckFingerprint


Another Use Case Example

Actor Bank Customer• Person who owns one or more Accounts in the

Bank.

Withdraw Money• The Bank Customer specifies a Account and

provides credentials to the Bank proving that s/he is authorized to access the Bank Account.

• The Bank Customer specifies the amount of money s/he wishes to withdraw.

• The Bank checks if the amount is consistent with the rules of the Bank and the state of the Bank Customer’s account. If that is the case, the Bank Customer receives the money in cash.


Use Case Attributes

Use Case Withdraw Money Using ATM

Initiatiating actor:• Bank Customer

Preconditions:• Bank Customer has opened a Bank Account with

the Bank and• Bank Customer has received an ATM Card and PIN

Postconditions:• Bank Customer has the requested cash or• Bank Customer receives an explanation from the

ATM about why the cash could not be dispensed


7. The Bank Customer inputs an amount.

3. The Bank Customer types in PIN.

5. The Bank Customer selects an account.

Use Case Flow of Events

1.The Bank Customer inputs the card into the ATM.

8.The ATM outputs the money and a receipt and stops the interaction.

4. If several accounts are recorded on the card, the ATM offers a choice of the account numbers for selection by the Bank Customer

6.If only one account is recorded on the card or after the selection, the ATM requests the amount to be withdrawn.

System steps

2.The ATM requests the input of a four-digit PIN.

Actor steps


Use Case Exceptions

Actor steps

1. The Bank Customer inputs her card into the ATM.[Invalid card]

3. The Bank Customer types in PIN. [Invalid PIN]

5. The Bank Customer selects an account .

7. The Bank Customer inputs an amount. [Amount over limit]

[Invalid card]The ATM outputs the card and stops the interaction.

[Invalid PIN]The ATM announces the failure and offers a 2nd try as well as canceling the whole use case. After 3 failures, it announces the possible retention of the card. After the 4th failure it keeps the card and stops the interaction.

[Amount over limit] The ATM announces the failure and the available limit and offers a second try as well as canceling the whole use case.


Guidelines for Formulation of Use Cases (1)

• Name• Use a verb phrase to name the use case.• The name should indicate what the user is trying to

accomplish.• Examples:

• “Request Meeting”, “Schedule Meeting”, “Propose Alternate Date”

• Length• A use case description should not exceed 1-2 pages. If

longer, use include relationships.• A use case should describe a complete set of

interactions.


Guidelines for Formulation of Use Cases (2)

Flow of events:• Use the active voice. Steps should start either

with “The Actor” or “The System …”.• The causal relationship between the steps

should be clear.• All flow of events should be described (not only

the main flow of event).• The boundaries of the system should be clear.

Components external to the system should be described as such.

• Define important terms in the glossary.


Example of a badly written Use Case

“The driver arrives at the parking gate, the driver receives a ticket from the distributor, the gate is opened, the driver drives through.”

• What is wrong with this use case?


Example of a badly written Use Case

“The driver arrives at the parking gate, the driver receives a ticket from the distributor, the gate is opened, the driver drives through.”

It contains no actorsIt is not clear which action triggers the ticket being issuedBecause of the passive form, it is not clear who opens the gate (The driver? The computer? A gate keeper?)It is not a complete transaction. A complete transaction would also describe the driver paying for the parking and driving out of the parking lot.


How to write a use case (Summary)

• Name of Use Case• Actors

• Description of Actors involved in use case

• Entry condition • “This use case starts when…”

• Flow of Events • Free form, informal natural language

• Exit condition • “This use cases terminates when…”

• Exceptions • Describe what happens if things go wrong

• Special Requirements • Nonfunctional Requirements, Constraints


Summary

• Scenarios:• Great way to establish communication with client• Different types of scenarios: As-Is, visionary, evaluation

and training

• Use cases• Abstractions of scenarios

• Use cases bridge the transition between functional requirements and objects.

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